Today I spent the afternoon installing the Farnam Battery Heating Pads I purchased from KTA-EV. What took the longest was lugging the batteries out from the basement out to the EV, then hoisting them into the EV.
This is my temperature control circuit for the heater pads. This is essentially a massive on/off switch that uses a temperature sensor set to a value that trips when it is reached, then it shuts off the AC power to the heaters. I have it set to 75F and it will reactivate at 71F. I have also a fuse, master on/off switch for the AC and a solid state relay that does the heavy work.
And here are the pads themselves, installed in the trunk. They are 35 watts each, and I have one under each battery.I did a test yesterday with the whole setup in the basement, and it got the pads up to 75F from 50F in about 3 hours. During this test I used a thermal probe between the battery and pad, now in the EV I am using the thermal sensor from my PakTrakr, which lays on top of battery 1. After I finished reinstalling everything, I turned on the battery warmers for 3.5 hours and the Paktrakr sensor recorded 58F from 49F. I insulated my batteries so there is little space for the heat from the pads to rise, except through the batteries themselves.
I have to neaten up the electrical cord for the pads, so I shut them off for the night, but tomorrow I will sort that out and take the EV out for another drive, then apply the pads. Right now it is 35F out and 58F in my batteries, so I am curious to see how my new insulation packing will work out and check out the temps in the morning.